An object of the present invention is a method of producing crosslinked hydrogels of a new type, characterized by increased tensile strength and partly also by a rubber-like elasticity, by high adhesion to substrates and other useful properties.
It is another object of the invention to avoid one or more drawbacks of the prior art.
Other objects and advantages of the invention will become more apparent from the following detailed disclosure and claims.
In furtherance of the invention it has been found that the raw materials used in production of the new hydrogels are cheaper than glycol methacrylates and glycol acrylates of the prior art.
It has now been established that gels produced from copolymers of acrylonitrile containing acrylamide and carboxylic groups, react with dilute solutions of carbonates, hydrogen carbonates, phosphates, chromates and other inorganic salts, acids and hydroxides, preferably alkali or ammonium salts which form insoluble salts with Zn.sup.2+ ions, so that the precipitate of the corresponding insoluble zinc compound is formed in the surrounding solution, whereas in the gel, the Zn.sup.2+ ions are substituted, in the first place, by water and by the ions of the alkali metals or by the ammonium ion. On the other hand, anions enter the gel much less readily, owing to the anions already present therein and which are partially bonded to the copolymers. In contrast to the polyacrylonitrile and the copolymers of acrylonitrile with hydrophobic monomers, such as the C.sub.1 -C.sub.3 lower alkyl esters of acrylic and methacrylic acid, the copolymers according to the present invention do not substantially form a surface film of coagulated polymer, which would otherwise prevent or slow down the further diffusion of ions, whereby instead of the gel of the Zn.sup.2+ salt of the corresponding copolymer, a no less elastic and strongly swelling in water hydrogel is formed simultaneously with the washing out of the zinc chloride. If the surface layer of the hydrogel becomes turbid owing to the formation of a precipitate of an insoluble Zn.sup.2+ compound the turbidity can easily be removed by using a bath of a dilute mineral acid, such as hydrochloric or nitric acid, and by dipping the hydrogel again into a salt bath which forms an insoluble salt with the Zn.sup.2+ ions. In this way, substantially perfectly clear, transparent crosslinked hydrogels are obtained, substantially free from zinc and other metals, which might have irritating or other harmful effects when the hydrogels are in contact with living tissue or mucous membranes. In the case of technical uses, a slight turbidity due to negligible amounts of insoluble Zn.sup.2+ salt does not substantially matter in most cases, so that the aforesaid intermediary treatment with a dilute mineral acid can generally be omitted.
Broadly speaking, the instant invention includes the provision of a method of producing insoluble crosslinked hydrogels by polymerizing 20-40% by weight of acrylonitrile or of a mixture thereof with up to 15 mol percent of other monomers copolymerizable therewith in absence of added crosslinking agents, in a concentrated aqueous solution of zinc chloride, for which some other salt may be partly substituted (i.e. up to about 40% by weight CaCl.sub.2, MgCl.sub.2 or NaCl), and thereafter carrying out a partial acidic hydrolysis of the polymer plasticized with aqueous ZnCl.sub.2 containing solution. The partial hydrolysis in homogeneous acidic medium is carried out either by heating the gel or by increasing the acidity thereof with or without heating. The copolymers are obtained in the form of an elastic gel containing about 20% to about 40% of the polymer. The polymers contain mostly nitrilic and amidic side groups, with a minor amount of carboxylic and acrylimide groups. The content of carboxylic and imidic groups increases with increasing temperature. Zinc chloride and other metal cations are then removed from the gel by treatment of the gel with a solution of a carbonate, hydrogen carbonate, chromate, phosphate, or other compatible agent which forms an insoluble precipitate with the zinc ions.
A minor part of acrylonitrile in the starting monomeric material can be replaced, for example, by up to about 15% molar, of monomers capable of copolymerizing with acrylonitrile, such as of acrylamide, methacrylamide, acrylic acid, methacrylic acid, sodium ethylene sulfonate, mixtures thereof or the like. The term partial acid hydrolysis as employed herein is meant to include a conversion of a part of nitrilic groups, first to amide groups which can be partly further hydrolyzed to carboxylic groups, a small amount of imide groups being simultaneously formed. The term homogeneous acid medium is meant to include gels of polyacrylonitrile and its products of partial hydrolysis, plasticized by aqueous zinc chloride containing solutions, acidified with an inorganic acid soluble therein without forming a precipitate. The expression "mainly nitrilic and amide side groups" means that said groups, both together are present in amounts of about 60 to 100%, (molar), preferably about 80 to 99%. The "minor amount of carboxylic and acrylimide groups" means said groups are present in the product in amounts of about 0 to 40% (molar), preferably about 0.1 to 20%.
More specifically the invention includes the provision of preparing insoluble crosslinked hydrogel copolymers by polymerizing to form a gel about 20 to about 40% by weight of monomeric material consisting predominantly of acrylonitrile in absence of added crosslinking agent in an aqueous salt solution containing zinc chloride as the main component thereof, partially hydrolyzing the thus obtained gel consisting of polyacrylonitrile or its copolymers with up to 15% (molar) of other monomer, plasticized with said salt solution, and removing salt therefrom by treating the gel with a dilute aqueous solution of an electrolyte, the anion of which forms a water-insoluble zinc compound until substantially all of the zinc ions have been removed from said gel and replaced by water, thereby forming an insoluble crosslinked hydrogel.
The insolubility in any solvent is caused by covalent crosslinking which can be explained, in absence of usual cross-linking agents added, only by chain transfer onto the monomer and polymer. Soluble, gel-free copolymers of this kind can be obtained only if the initial acrylonitrile concentration is lower than 15% by weight; at 20% the product is sparingly but markedly crosslinked and therefore insoluble. Higher initial concentrations than 35% up to about 40% by weight of acrylonitrile are feasible, only the dissipation of the heat of polymerization is more difficult, and the shrinking is rather high.
The crosslinking by chain transfer in absence of any added crosslinking agents (usually a monomer with more than one polymerizable double bond), is caused by increasing the initial acrylonitrile concentration to 20-40% by weight, related to the weight of the polymerization batch as a whole. Zinc chloride solutions do not cause chain transfer onto the solvent. It is understood, naturally, than no compound causing chain transfer is present as an impurity or added intentionally, because in such a case the chain transfer onto the monomer would be either reduced or fully avoided. The degree of crosslinking depends on the initial acrylonitrile concentration, increasing proportionally therewith.
The crosslinking by the chain-transfer onto the monomer posseses considerable advantages. First, the crosslinks possess the same chemical composition as the main chains, containing no easily hydrolyzable or chemically attackable chain links. Second, the crosslinks are, in average, considerably longer than in case of using a common crosslinking agent such as ethylene glycol dimethacrylate. This can be explained by the fact that chain-transfer onto the monomer results in a trifunctional co-monomer having one function, namely the free radical, more reactive than the two other functions, namely the double bond. Thus, a sufficiently long polymer chain has grown on the free radical prior to the reaction of the double bond with another growing free radical. Moreover, the crosslinks are formed by a recombination of two growing free radicals so that their final length is still longer.
The absence of easily hydrolyzable chain links results in increased chemical stability, the presence of long crosslinks improves physical and physico-chemical characteristics of the product (elasticity, strength, swelling capacity, structural strength and the like).
The polymerization of acrylonitrile in an aqueous solution of an inorganic salt, the main component of which is formed by zinc chloride, followed by partial acid hydrolysis has in this case many advantages. Firstly, a concentrated aqueous solution of zinc chloride is a good solvent for acrylonitrile, as well as for its polymers and copolymers formed following the partial hydrolysis. Organic solvents, such as dimethylformamide or dimethylsulfoxide, are not well suited for this purpose, since they lead to a considerable chain transfer, thus largely reducing the degree of polymerization and are further inflammable or poisonous, and still further their recovery is comparatively difficult. Inorganic salts, on the other hand, exhibit only a negligible chain transfer in free radical polymerization, so that chain transfer onto the monomer leads to branching and crosslinking. Moreover, said salt solutions have the advantage that they substantially do not dissolve molecular oxygen, so that in most cases no inhibition is observed, even if the polymerization is not carried out under an inert gas. The method according to the present invention provides for virtually complete recovery of the solvent in the form of an insoluble salt, as almost no zinc is present in the waste water. In this way, the most difficult problem of waste water pollution is solved, with the additional advantage of the recovery of a metal in a time of shortages. If a countercurrent system of washing is employed, only a small amount of the salt used for the precipitation is transferred into the waste water; and if a harmless salt, such as sodium hydrogen carbonate has been used, the waste water pollution is negligible.
The electrolytes used for the precipitation of zinc cations can be chosen in such a way as to yield zinc in the form of a white or coloured pigment or raw material suitable for being used in pigment preparation. This again increases the economy of the process.